The present invention relates to electro-vibration transducers, and more particularly to an electro-vibration transducer employed as a vibrating source for a body-sensible acoustic vibration device.
A body-sensible acoustic vibration device has a special electro-vibration transducer which converts sound signals from an acoustic device such as a loudspeaker and also low-frequency sounds in a frequency range lower than the audio frequency range into mechanical vibrations of a member with which the body of the listener is in contact, thereby allowing the listener to sense the sounds directly through his or her body as if he or she were present at a live performance.
There have been proposed in the art a variety of electro-vibration transducers. One example of such a transducer is shown in FIG. 1. The transducer is installed on the frame of a chair, for instance.
The electro-vibration transducer has a cylindrical case 1 made of resin with both ends closed. An annular magnet 2 is arranged in the case 1 and secured to the latter with a damper 3 made of a leaf spring material or the like. An annular yoke plate 4 and a yoke 5 are coaxially secured to respective upper and lower surfaces of the magnet 2. The yoke 5 has a pole 5a extending from its center. The pole 5a, the magnet 2 and the annular yoke plate 4 form a magnetic gap 6. A cylindrical bobbin 8 is inserted into the magnetic gap 6 and is fixedly secured to a closed end 1a of the case 1. A voice coil 9 is wound on the outer cylindrical wall of the bobbin 8.
In the electro-vibration transducer thus constructed, the damper 3 is made of a material such as a leaf spring material having a small internal loss. Therefore, the resonance sharpness Q at the low resonance frequency f.sub.0 is large, as indicated by the solid line in FIG. 2, with the result that the effective bandwidth of the device is narrow and its transient response is poor.
In order to overcome these difficulties, a variety of methods have been employed. Among these are methods of improving the transducer by providing a visco-elastic member between the case 1 and the magnetic circuit, filling the magnetic gap 6 with magnetic fluid, and employing a compound member as the damper 3. These improvements can succeed in decreasing the resonance sharpness, as indicated by the broken line 10b in FIG. 2, thus increasing the effective bandwidth and improving the transient response. However, the resulting effective bandwidth is still not wide enough.
In order to eliminate the above-described difficulties accompanying the transducer shown in FIG. 1, an electro-vibration transducer as shown in FIG. 3 has been proposed. As is apparent from FIG. 3, the transducer is formed by adding a magnetic circuit and a voice coil to the electro-vibration transducer shown in FIG. 1. The magnetic circuit is composed of a magnet 12 secured through a damper 11 to the case 1, and an annular yoke plate 14 and a yoke 15 fixedly secured to the magnet 12. A bobbin 18 on which the voice coil 19 is wound is inserted into a magnetic gap 16 formed in the magnetic circuit. In FIG. 3, those components which correspond to similar components in FIG. 1 are designated by the same reference numerals.
This transducer is so designed that, as shown in FIG. 4, the low resonance frequencies of the two driver units, namely, the magnetic circuits, are set to suitable values f.sub.01 and f.sub.02 so that the effective bandwidth is sufficiently widened with the bandwidth between the two values f.sub.01 and f.sub.02 acting as an apparent passband. However, since the transient response cannot be improved without decreasing the resonance sharpness at the low resonance frequencies, the same improvements effected to the transducer shown in FIG. 1 can be applied to the transducer shown in FIG. 3, for instance, the magnetic gaps 6 and 16 can be filled with magnetic fluid.
With this electro-vibration transducer, a sufficiently wide effective bandwidth and a satisfactory transient response can be obtained. However, the transducer suffers from a difficulty that the leakage flux of the two magnetic circuits affects the vibration response of the magnetic circuits, making it nonlinear.